Abstract

Almost all of the depolarization papers in the lidar literature employ a physically inappropriate notation and they use a definition of the depolarization ratio that is not linear in the quantity of interest. This depolarization lidar legacy is misleading and confusing. In particular, subscripts meaning parallel and perpendicular do not apply to atmospheric parameters, such as the volume backscatter coefficient, because (for linear polarization) the two components of the backscattered light are polarized in the transmitted sense and completely unpolarized; the unpolarized component is not “perpendicular.” An analysis of lidar depolarization measurements with a particle scattering matrix recently provided in the literature yields algorithms for retrieving the depolarization parameter from either linear or circular depolarization lidar measurements. The analysis, notation, and definitions recommended here harmonize lidar depolarization analysis with radiative transfer theory, particle scattering theory, and standard polarization measurement techniques.

Figures (3)

Schematic illustration of a linear lidar depolarization measurement. The transmitted polarization is linear along the x axis (horizontal, in this figure). The received power is analyzed in two receiver channels with linear polarization analyzers oriented horizontally (parallel) and vertically (perpendicular). The powers emerging from the analyzers are converted to signals by detectors. For single scattering by spheres, all of the received power is in the parallel receiver channel.

Schematic illustration of a circular lidar depolarization measurement. The transmitted polarization is linear at +45° and it passes through a quarter-wave plate with the fast axis vertical to form RHC. The received power passes through quarter-wave plates with the fast axes vertical and the linear analyzers at +45° and −45°. The powers emerging from the analyzers are converted to signals by detectors. The signal from the +45° analyzer is called copolar and the signal from the −45° analyzer is called cross-polar. For single scattering by spheres, all of the received power is in the copolar receiver channel.